1. Field of the Invention
The present invention relates generally to a de-inking process and more specifically, to a closed floatation conventional de-inking process adopted in the recycled paper processing plants, in which, ink separation and removal constitutes a major task. The specific concerns of this invention are the concept and the equipment for the separation and removal of the ink and ash from the pulp slurry contaminated with ink, clay and stickies.
Waste paper, usually called by the public, is actually not a waste and a very valuable source of fiber for recycled paper mills. Recovery of fiber has been increasing due to the solid waste problem and the demand by industry as well. There are several kinds of recovered paper, such as old corrugated container, old news print, white grades with several sub categories. The old corrugated container is usually recycled with out any ink removal to produce brown grade paper and boards.
A majority of the old newsprint and other white grade paper are recycled into various valuable paper categories using the de-inking process. There are two types of de-inking called: washing and flotation. Flotation usually has a washing step in the subsequent steps and can be called a combination process. The washing de-inking without any ink purging using some kind of white water clarification is not true de-inking as the ink recycles in the system. It is less common to have washing as the sole de-inking process at present. The washing process does not help in handling large ink and other dirt particles whereas the flotation process separates and removes a wide range of particles including small particles.
Several types of chemistries are used in the de-inking process like hardness dependent calcium soap froth, hardness independent surfactant, enzymatic and so forth. All these chemistries help release the ink from the fiber and attach the ink particles to the air bubbles in a selective adsorption process. The process needs equipment to mechanically separate and remove the ink off the pulp suspension. This is commonly called a flotation cell in the de-inking mills.
A de-inking mill usually has re-pulping equipment called a pulper at the beginning of the process to repulp the recovered paper brought into the mill for recycling. The re-pulping is done with water and chemicals to aid the pulping and sometimes bleaching at the same time. De-inking agent is usually added in the pulper and gets well dispersed with the pulp. The ink release from the fiber is aided by the de-inking agent with the wetting of the fiber surface as well as swelling. The same surfactant helps in the froth flotation in the primary de-inking system for the formation of the foam and selective adsorption of the ink particles onto the air bubbles.
For the effective separation of the ink from the pulp suspension, the air bubbles should be formed mechanically inside the pulp suspension which raise to the surface on which the ink particles adsorb to float to the surface.
A flotation cell is very important in bringing air and dispersing the air to increase the collision frequency with the ink particles along with other unwanted hydrophobic materials. The higher the bubble-ink collision frequency the better the chances of ink attachment to the foam. Attaching as much ink as possible under given operating conditions flotation cells remove the foam using several techniques like overflow, scraping using paddles, pressure under enclosed vessel and so forth. Thus de-inking is accomplished in various ways within froth flotation technology.
The closed floatation conventional de-inking process for recycled paper of the present invention is used to separate the ink, clay, stickies and pulp fines from the recycled pulp slurry. The contaminated pulp slurry is injected into the flotation cell and the foam generated is drawn out of the cell by the vacuum produced by the injector using a separator.
A shower is used in the separator to flush the slurry down on a periodic basis. A moisture trap is used after the separator to trap any moisture in the air before recycling the air back in the injector.
The air flow control system comprising a safety valve is used to add extra air from the atmosphere in case of loss of air. A flow meter is used to measure the flow of air drawn by the injector. An exhaust line off the moisture trap connected to the flow meter recycles the air preventing the humidity build up in the plant.
The reject from the separators is drained into the effluent stream and the accept from the flotation cell is sent to the downstream process.
2. Description of the Prior Art
There are other processes for de-inking pulp. Typical of these is U.S. Pat. No. 3,098,784 issued to Gorman, Jr. on Jul. 23, 1963.
Another patent was issued to Illingworth, et al. on Nov. 21, 1967 as U.S. Pat. No. 3,354,028. Yet another U.S. Pat. No. 4,328,095 was issued to Ortner, et al. on May 4, 1982 and still yet another was issued on May 25, 1982 to Bamscheidt as U.S. Pat. No. 4,331,534.
Another patent was issued to Krofta on Sep. 7, 1993 as U.S. Pat. No. 5,242,585. Yet another U.S. Pat. No. 5,310,459 was issued to Krofta on May 10, 1994. Still yet another U.S. Pat. No. 5,273,624 was issued on Dec. 28, 1993 to Chamberlain, et al Another was issued to Serres on Mar. 6, 2001 as U.S. Pat. No. 6,197,153 and another U.K Patent No. GB2017780 to Ortner, et al. on Oct. 10, 1979.